Semiconductor lasers fabricated from strained layer quantum well material have attracted considerable interest as pump lasers for erbium doped glass fiber amplifiers. However, ridge waveguide lasers with planar multi-quantum well (MQW) In.sub.0.2 GA.sub.0.8 As/GaAs heterostructures grown by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) growth techniques are weakly index-guided, and their mode stability is not expected to be as good as that for a strongly index-guided buried heterostructure lasers. For example, see N. K. Dutta et al. "Temperature Dependence of Threshold of Strained Quantum Well Laser", Appl. Phys. Lett. 58 (11), Mar. 18, 1991, pages 1125-1127. The latter include several types of InGaAs/GaAs buried heterostructure lasers fabricated using multistep growths. An improvement in the lateral optical confinement in quantum well stripes resulted in lasers with GaAs/AlGaAs heterostructures fabricated using a single step growth on nonplanar GaAs substrates. One example of such a single growth structure is disclosed in an article by D. E. Ackley and G. Hom entitled "Twin-Channel Substrate-Mesa Guide Injection Lasers Fabricated by Organometallic Vapor Phase Expitaxy", Applied Physics Letters, Vol. 42, No. 8, April 1983, pages 653-655. Ackley and Hom disclose a high-power index-guided laser fabricated by a one-step organometallic vapor phase epitaxy on photolithographically patterned and etched substrates. A narrow mesa is defined by a twin-channel v-groove structure. The twin channel structure provides for an improved control over the width of the lateral waveguide relative to a single groove configuration, which allows well-controlled zero order mode operation. However, the growth of the active layer is continuous over the side walls of the v-grooves of the twin-channel defined mesa structure. This would lead to the spreading of carriers across the whole of the width of the active layer, not just across its portion on the mesa. Lateral electrical confinement was achieved by ion implantation after the growth of the semiconductor structure. Since ion-implantation requires subsequen heating of the device, this adds to the complexity of the process, negating the advantages of the single-step growth processing. Therefore, it is desirable to produce a buried heterostructure laser in a more simplified manner.